Embodiments herein generally relate to corrections made to trap areas that are added between abutting objects on an image to be printed by a printing engine.
The art of color printer calibration has been well studied, the applications of which can be found in several consumer products today. Most of these methods employ the Neugebauer approach, whereby a series of uniform area patches is measured to develop a set of equations to describe the reflectance of a halftone pattern on paper. This set of equations, denoted the “printer model” is then inverted to determine one or more lookup tables, used to produce the desired printer output. These lookups will generally produce smooth monotonic output for each channel, and for CMYK (cyan, magenta, yellow, black) printers there may be an additional requirement to achieve gray balance between (CMY) and K. In many marking systems, in particular those involving electrophotography, the ink/toner densities at halftoned edges can be quite different relative to uniform regions.
As described in U.S. Patent Publication 2004/0114162, to McElvain, incorporated herein by reference, electronic processing of graphic and text images produces multi-color prints using multiple color separations. Typically, four process colors, cyan, magenta, yellow and black, are used to print multiple separations, which tend to have minor misregistration problems. The result of abutting or overlapping shapes is a boundary between adjacent regions of color that, under ideal printing conditions should have zero width. That is, one color should stop exactly where the abutting color begins, with no new colors being introduced along the boundary by the printing process itself. The “colors” which fill the shapes can be solid colors, tints, degrades, contone images, or “no fill” (i.e., the paper with no ink applied). In general, the “colors” represented in these adjacent regions are printed using more than one colorant. In practice therefore, the realization of a zero width boundary between regions of different color is impossible as a result of small but visible misregistration problems from one printed separation to another. The error is manifested as a “light leak” or as a visible boundary region of an undesired color.
Methods for correcting for this misregistration are known. The general approach is to expand one of the abutting regions' separations to fill the gap or misregistration border region with a color determined to minimize the visual effect when printed. Borders or edges expanded from a region of one color to another in this manner are said to be “spread”. A border which has been expanded is referred to as a “trap”, and the zone within which color is added is called the “trap zone”. Edge detection and image manipulation to perform trapping may be done in any of several processes, including for example, the technique described in U.S. Pat. No. 6,345,117 to Klassen, incorporated herein by reference.
Certain marking systems produce variations in desired output color from print engine to print engine. These variations are due to specific physical characteristics of the particular print engine. A common technique for correcting for variations in color output is to measure a set of printed colors against a set of control colors and to provide a lookup table generated from an analytic function (such as a gamma correction function) to correct for variations in color output. Thus, all print engines of a particular model can be corrected to have the same desired output color.
Certain marking systems exhibit difficulty maintaining color uniformity near edges of abutting objects on the image. For example, it may be intended to place a 75% fill, which may be possible in the body of the fill object; however, within a few millimeters of the edges, more or less than 75% may be deposited, depending on the marking process. For trapping, this can be a significant problem, as the trapping operation fundamentally only modifies edge pixels of abutting objects. If the trapping engine specifies a trap color (for a given separation) of 35%, only 20% may be produced as a result of the edge physics, for example. It is therefore desirable to have the capability to correct for these edge modulations, in order to achieve traps that are more pleasing to the eye.
The current approach to resolving this problem is to apply a gamma correction to the trap colors (gamma<1), such that the trap colors are generally forced toward the darker of the two abutting colors (see U.S. Patent Publication 2004/0114162, to McElvain). This empirical approach, while providing nominally better trap color rendering in the midtones relative to no correction, can produce excessively dark traps in many cases. Furthermore, the correction percentage would need to change based on the trap width, since the printed color should be equal to the intended color as the trap width becomes large. Therefore, it would be desirable to have a correction mechanism that is robust across all colors and trap widths, and accounts for the physical behavior of the marking process at the edges.
For xerography, average edge tangential electric fields can have a significant effect on the density of the toner cloud, as well as its proximity to the photoreceptor. Likewise at edges, field components parallel to the PR surface can give rise to toner cloud displacements, and can result in artifacts such as lead-edge and trail-edge deletions. Both these factors are manifested in the form of reduced dot gain, and partial dots at edges will be either significantly reduced in size or completely eliminated. Clearly, modeling each of these edge nonlinearities from a theoretical standpoint is a formidable task, and such a model would not be amenable to applications that require real time corrections.
Edge processing processes such as trapping and anti-aliasing are most affected by these nonlinearities. As illustrated in FIG. 1, a thin strip of an intermediate digital value (C) is placed at the position of the original intersection between two patches (A and B). Because of the edge marking nonlinearities, the actual printed toner densities in these regions can be strongly distorted, and can lead to objectionable artifacts. Clearly these effects will be compounded for intersections involving multiple color separations. In the case of trapping, an intentional multi-pixel “defect” is added at color interfaces to mitigate registration errors, where the width of the trap is determined by the maximum misregistration of the marking system. Generally, the color of this defect is chosen such that it is not objectionably visible in the presence of the original two intersecting colors. For marking processes such as xerography, the printed color of the trap region may take on an undesirable color cast in the presence of nonlinearities, thus reducing the benefit of trapping. It would therefore be desirable to apply corrections to these regions to produce the colors that were originally intended.
As previously mentioned, a correction process involving a complete description of the edge development/transfer physics may provide an accurate edge color correction, but would generally not be practical for applications that require real time correction. On the other hand, a truly empirical correction process that does not include details of the edge behavior might be computationally efficient, but would most likely provide inadequate correction in certain regions of color space.